1. Field of the Invention
This invention relates to a range of cardiovascular and other implants fabricated of metallic alloys of enhanced hemocompatibility that can optionally be surface hardened to provide resistance to wear, or cold-worked or cold-drawn to reduce elastic modulus, if necessary. More specifically, the invention is of synthetic heart valves, ventricular assist devices, total artificial hearts, stents, grafts, pacers, pacemaker leads and other electrical leads and sensors, defibrillators, guide wires and catheters, and percutaneous devices fabricated of Ti-Nb-Zr alloys.
2. Description of the Related Art
Cardiovascular implants have unique blood biocompatibility requirements to ensure that the device is not rejected (as in the case of natural tissue materials for heart valves and grafts for heart transplants) or that adverse thrombogenic (clotting) or hemodynamic (blood flow) responses are avoided.
Cardiovascular implants, such as heart valves, can be fabricated from natural tissue. These bioprostheses can be affected by gradual calcification leading to the eventual stiffening and tearing of the implant.
Non-bioprosthetic implants are fabricated from materials such as pyrolytic carbon-coated graphite, pyrolytic carbon-coated titanium, stainless steel, cobalt-chrome alloys, cobalt-nickel alloys, alumina coated with polypropylene and poly-4-fluoroethylene.
For synthetic mechanical cardiovascular devices, properties such as the surface finish, flow characteristics, surface structure, charge, wear, and mechanical integrity all play a role in the ultimate success of the device. For example, typical materials used for balls and discs for heart valves include nylon, silicone, hollow titanium, TEFLON.TM., polyacetal, graphite, and pyrolytic carbon. Artificial hearts and ventricular assist devices are fabricated from various combinations of stainless steel, cobalt alloy, titanium, Ti-6Al-4 V alloy, carbon fiber reinforced composites, polyurethanes, BIOLON.TM. (DuPont), HEMOTHANE.TM. (Sarns/3M), DACRON.TM., polysulfone, and other thermoplastics. Pacers, defibrillators, leads, and other similar cardiovascular implants are made of Ni-Co-Cr alloy, Co-Cr-Mo alloy, titanium, and Ti-6A1-4 V alloy, stainless steel, and various biocompatible polymers. Stents and vascular grafts are often made of DACRON.TM. stainless steel or other polymers. Catheters and guide wires are constructed from Co-Ni or stainless steel wire with surrounding polymer walls.
One of the most significant problems encountered in heart valves, artificial hearts, assist devices, pacers, leads, stents, and other cardiovascular implants is the formation of blood clots (thrombogenesis). Protein coatings are sometimes employed to reduce the risk of blood clot formation. Heparin is also used as an anti-thrombogenic coating.
It has been found that stagnant flow regions in the devices or non-optimal materials contribute to the formation of blood clots. These stagnant regions can be minimized by optimizing surface smoothness and minimizing abrupt changes in the size of the cross section through which the blood flows or minimizing either flow interference aspects. While materials selection for synthetic heart valves, and cardiovascular implants generally, is therefore dictated by a requirement for blood compatibility to avoid the formation of blood clots (thrombus), cardiovascular implants must also be designed to optimize blood flow and wear resistance.
Even beyond the limitations on materials imposed by the requirements of blood biocompatibility and limitations to designs imposed by the need to optimize blood flow, there is a need for durable designs since it is highly desirable to avoid the risk of a second surgical procedure to implant cardiovascular devices. Further, a catastrophic failure of an implanted device will almost certainly result in the death of the patient.
The most popular current heart valve designs include the St. Jude medical tilting disc double cusp (bi-leaf) valve. This valve includes a circular ring-like pyrolytic carbon valve housing or frame and a flow control element which includes pyrolytic carbon half-discs or leaves that pivot inside the housing to open and close the valve. The two leaves have a low profile and open to 85.degree. from the horizontal axis.
Another popular heart valve is the Medtronic-Hall Valve wherein the flow control element is a single tilting disc made of carbon coated with pyrolytic carbon which pivots over a central strut inside a solid titanium ring-like housing. A third, less popular design, is the Omniscience valve which has a single pyrolytic disc as a flow control element inside a titanium housing. Finally, the Starr-Edwards ball and cage valves have a silastic ball riding inside a cobalt-chrome alloy cage. The cage is affixed to one side of a ring-like body for attachment to the heart tissue. More recent designs include trileaflet designs and concave bileaflet designs to improve blood flow.
From the point of view of durability, heart valves made of low-thrombogenic pyrolyte carbon could fail from disc or pivot joint wear or fracture related to uneven pyrolytic carbon coating, fracture of the ball cage, disc impingement, strut wear, disc wear, hinge failure, and weld failure. A more recent heart valve, the Baruah Bileaflet is similar to the St. Jude design but opens to 80.degree. and is made of zirconium metal. The valve has worked well over its approximately two-year history with roughly 200 implants to date in India. This performance can be partly attributed to the lower elastic modulus of zirconium (about 90 GPa) and the resultant lower contact stress severity factor (Cc of about 0.28.times.10.sup.-7 m) when the disc contacts the frame. In contrast, pyrolytic constructions produce contact stress severity factors of about 0.54.times.10.sup.-7 m.
Although zirconium has worked well to date and can reduce contact stress severity, zirconium metal is relatively soft and sensitive to fretting wear. This is partly due to hard, loosely attached, naturally-present passive oxide surface films (several nanometers in thickness) which can initiate microabrasion and wear of the softer underlying metal. However, this naturally present zirconium oxide passive film is thrombogenically compatible with blood and the design is acceptable from a hemodynamic standpoint. Therefore, while the zirconium bileaflet valve appears to meet at least two of the major requirements for cardiac valve implants, namely blood compatibility and design for minimum stagnant flow regions, the use of soft zirconium metal leads to a relatively high rate of fretting wear and leads to the expectation that the valve may be less durable than one produced from materials less susceptible to fretting wear. Titanium and titanium alloys present a similar limitation, and Co-Cr-Mo, stainless steel, and Co-Ni alloys have much greater elastic modulus.
There exists a need for a metallic cardiac valve implant that is biocompatible, compatible with blood in that it does not induce blood clotting and does not form a calcified scale, that is designed to minimize stagnant flow areas where blood clotting can be initiated, that has a low elastic modulus for lower contact stress severity factors to ensure resistance to wear from impact, and that has a surface that is also resistant to microabrasion thereby enhancing durability.
Heart diseases, many of which cannot be cured by conventional surgery or drug therapy, continue to be a leading cause of death. For the seriously ill patient, heart replacement is often one of the few viable options available.
In 1988, the NHBLI began funding research and development for permanently implantable, electrically driven, total artificial hearts (TAHs). The pumping mechanism of the TAHs would be implanted into the chest cavity of the patient and the device would be powered by a battery pack and a small transformer, worn by the patient, which transmits energy to the heart with no physical connections through the skin. The NHBLI funded four separate groups: The Cleveland Clinic Foundation & Nimbus, Inc.; Pennsylvania State University & Sarns/3M; The Texas Heart Institute and Abiomed, Inc.; and the University of Utah. Consequently, four competing designs were developed.
The development of TAHs posed several issues. Firstly, it was necessary to duplicate the action of a human heart, ensure long-term reliability and biocompatibility, while producing a device that fits into the chest cavity in terms of both its total volume and the orientation of its connections to natural vessels in the body. Aside from the purely mechanical, wear, and power supply issues, it is also necessary that the design and materials prevent infection and thrombosis. Blood is a non-Newtonian fluid and its properties, such as viscosity, change with oxygen content, kidney function, and even the age of the patient. Further, plasma contains a suspension of fragile red blood cells which may be caught in artificial valves, or other mechanically stressful areas, thereby destroying these cells. It is therefore necessary to develop a TAH that does not stress blood components, and to fabricate the pump from materials that are not only biocompatible, but also "blood compatible" in the sense of minimizing damage to blood components and minimizing the formation of blood clots.
Many of the above comments also apply to ventricular assist devices (VADs), one of which is being developed by the Novacor Division of Baxter Health Care Corp. In the use of a VAD, the patient's heart remains in place while the VAD boosts the pumping pressure of the left ventricle of the heart. Consequently, the VAD is an assist device rather than a replacement. However, the VAD must be blood compatible for the same reasons as the total artificial heart.
There exists a need for a material that is lightweight, readily formable into complex shapes, biocompatible, and blood and tissue compatible with a hard surface that is resistant to abrasive wear, microfretting wear, and the corrosive effects of body fluids, for use in heart assist or replacement devices (including EMHs, VADs, and TAHs) to prolong the life of mechanical components while at the same time minimizing any deleterious effect on blood components.